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The Effect of Stump-Muscle Activity on Socket Suspension and Prosthetic Control: A Preliminary Report

In modern amputation surgery maximum retention and stabilization of stump muscles are essential4,5. When combined with immediate postsurgical prosthetics fitting and the training of stump musculature, this type of amputation results in an effective, functional substitute end-organ for which an appropriate prosthesis serves as an interface to the environment. Weiss refers to the surgery which produces a muscle-siabilized stump as a "physiological amputation"8. Murdoch has described such a stump as being "strong and stiff""6,7. As the standards for amputation surgery are upgraded throughout the world, it would be reasonable to anticipate that the muscle-stabilized physiological amputation would become the rule rather than the exception1.

For some time it has been observed clinically at the Prosthetics Research Study (PRS) that a strong, muscular stump provides improved prosthetic suspension capability. The design of the prosthesis and its method of suspension can be modified to take advantage of these circumstances.

This preliminary report presents information on attempts to determine the suspension ability of muscle-stabilized, below-knee slumps using muscular contraction as the primary means of socket suspension. Eleven unilateral below-knee amputees were evaluated. The physical dimensions of the stump were measured with the patient in repose, i.e., resting.

These circumferential measurements were then repeated with the patient exerting maximum voluntary muscle contraction. Differences in slump size and configuration were then studied ( Fig. 1 and Fig. 2 ). Pressure transducers were used to record pressure changes between the prosthesis and the stump in the areas of the maximum muscle bulge of the gastrocnemius and the anterior tibial muscle groups. Force readings to determine the amount of pull necessary to remove the prosthesis from the slump at different degrees of knee flexion were made with and without voluntary muscle contraction. Electromyographic readings of stump-muscle activity were recorded; and. finally, time measurements taken during the gait cycle were used to compare the natural with the amputated leg while the patient was using (1) the prescribed suspension, and (2) the inherent muscle suspension ability of the stump with the suspensory device or devices removed.

The 11 subjects tested ranged in age from 22 to 77 years. Their amputations resulted from trauma and its complications, or from peripheral vascular disease. Two amputations were performed less than two weeks following injury. Six additional amputations were done as the result of infection and drainage attributable to injuries occurring two to ten years prior to the date of limb ablation. The other three amputations resulted from peripheral vascular disease, one patient having undergone ten revascularization procedures before the final definitive below-knee amputation (Chart 1 ).

Patellar-tendon-bearing, hard-socket, cuff-suspension prostheses were fitted to four of the patients. Four additional amputees wore suprapatellar, supracondylar, hard-socket limbs with wedge suspension. One amputee had a similar suspension with the patellar area of the socket cut away, i.e., a supracondylar fitting. The remaining two amputees wore Kemblo-insert prostheses, one suspended by side joints and a lacer; the other, a supracondylar limb-incorporating wedge suspension as part of the insert. Stump length varied from five to seven inches. All amputees had undergone the PRS type of myoplastic closure. Myodesis was not performed on any of these patients.

Circumferential measurements of the stump were taken at the point of maximum increase of both the gastrocnemius and anterior tibialis muscles. With the patient in a sitting position and the knee in 20 deg. of flexion, the resting gastrocnemius muscle measurements ranged from 10 1/2 to 12 3/8 in. When the muscle was contracted maximally, the measurements increased an average of approximately 3/8 in. They ranged from 10 3/4 to 12 5/8 in. The smallest increase was 1/8 in. and the largest was 1/2 in.

Circumferential measurement at the level of the anterior tibialis when relaxed varied from 10 1/8 to 13 1/8 in. Individual increases from relaxed to the contracted state were from 0 to 1/4 in. (Chart 2 ).

In an attempt to determine the quality of static limb suspension, each amputee was seated comfortably with his prosthesis fitted. A scale was attached to the foot of the prosthesis and a direct-line pulling force was exerted on the limb when the stump muscles were relaxed and when they were contracted. This pull was exerted with the knee at 10, 45, and 90 deg. of flexion, respectively. The force necessary to displace the prosthesis 1 in. distally with the stump muscles relaxed and the knee in 10 deg. of flexion was from 5 to 18 lb. Under identical conditions with the stump muscles contracted, the range was from 8 1/2 to 50+ lb. (Chart 3 ).

When the knee was placed in 45 deg. of flexion and muscles relaxed, the tension needed to pull the prosthesis 1 in. distally varied from leg weight (approximately 3 to 4 lb., except Case Nos. 2, 6, 7, and 8 (PTS sockets) and Case Nos. 10 and 11 (PTS sockets with insert wedge)) to 18 lb. When the stump muscles were contracted, the ranges were increased from leg weight to 60+ lb. (Chart 3 ).

With the knee in 90 deg. of flexion and the muscles relaxed, the range to displace the prosthesis 1 in. varied from leg weight to 25 lb.; when the muscles were contracted, the range to pull 1 in. distally was leg weight to 60+ lb. (Chart 3 ).

In an effort to record the changes in pressure between the socket and the maximum muscle bulge of the gastrocnemius and the anterior tibialis muscles, pressure transducers were placed on the stump in the areas of maximum anterior tibialis and gastrocnemius contraction. The patients were instructed to contract the gastrocnemius during stance phase-primarily from foot-flat to toe-off. During the stance phase of gait the pressure at the gastrocnemius site with the prescribed suspension, and then with suspension removed and only muscle contraction for suspension, increased between 4 and 14 lb. The increases in anterior tibialis pressure in relation to muscle suspension versus prescribed suspension were not as significant. The anterior tibialis was contracted from toe-off through heel-strike. Initial increases of from 2 to 4 lb. were recorded. However, these increases did not carry through the entire swing phase.

Electromyographic recordings were made during ambulation. Signals from the anterior tibialis muscle as well as the gastrocnemius were recorded. This technique must be further refined before any conclusive information can be obtained. The surface-electrode readings obtained, while not consistent, did indicate a significant increase in activity when the amputee actively used his muscles as an aid in suspending the prosthesis, as against walking with the prescribed suspension and his normal gait.

To determine any deterioration or improvement in gait while using the prescribed suspension as against muscle contraction suspension, the amount of time spent on the amputated and normal legs was measured. These data were recorded in terms of the time spent on the prosthetic and normal legs during stance phase with the prescribed suspension, then again with only muscle-contraction suspension.

Complete data were compiled on nine patients. When regular suspension was used, six patients spent more time on their normal leg and three patients spent an equal amount of time on the normal and the prosthetic legs. With muscle suspension, three patients spent more time on the normal leg, one patient spent more time on the prosthetic leg, and five patients spent an equal amount of time on the normal and prosthetic legs.

From a subjective point of view, the patients tested varied in their feeling of security between walking without their prescribed suspension and relying only on muscle to stabilize and suspend the limb. All but one felt that they walked at least as well, and some felt more secure and comfortable when using only muscle suspension as compared to using their prescribed suspension.

All demonstrated some stump-muscle activity even when wearing the prescribed suspension. With the suspension removed, all felt they were using their muscles more actively and were conscious of "muscle-socket grasping."

In nine of eleven subjects the amount of visible piston action was no greater with muscle suspension alone than with the prescribed suspension.

All patients felt that their balance was improved when they used muscle suspension.

Conclusions

Additional suspension assistance is provided by the length and configuration3 of the myoplastic muscle-stabilized stump, especially when the medial and lateral gastrocnemius borders cause a slight flaring effect on the distal stump. These muscular "ears" or corners2 fit well into the socket configuration designed for them and provide some degree of suspension. These stumps are cylindrical rather than tapered.

Decreased reliance on the prescribed suspension will result in improved muscle use and thereby improve muscle suspension. Too much emphasis can be placed on snug suspension devices and thus diminish the capability of the muscles to work normally to improve suspension. The below-knee socket and suspension systems must be designed with awareness of the muscle-suspending potential of the stump, and provide an atmosphere where muscles can contract against the socket wall. Care must be taken to keep the ply of stump socks to a minimum, preferably not more than 6 and 10 ply.

Since it is possible to suspend a prosthesis through the mechanisms of stump length, stump configuration, and active muscle contraction, it is necessary to instruct the patient concerning the use of his stabilized muscles in a normal fashion in order to take advantage of these natural aids to improve balance and gait.

These studies will be continued. Further in-depth observations will be directed to the improvement of surgery, prosthetics design, and the effects of stump-muscle training.

8. Weiss, M., A. Gielzynski, and J. Wirski, Myoplasty-Immediate Fitting-Ambulation International Society for Rehabilitation of the Disabled, New York. (Reprint of paper presented at the Sessions of the World Commission on Research in Rehabilitation, 10th World Congress of the International Society, Wiesbaden, Germany, September 1966.)